Surface composition and dynamical evolution of two retrograde objects in the outer solar system: 2008 YB3 and 2005 VD

3 MB·English

by  N. Pinilla-Alonso

Astronomy&Astrophysicsmanuscriptno.2008YB3-PinillaAlonso (cid:13)c ESO2013
January11,2013
Surface composition(cid:63) and dynamical evolution of two retrograde
objects in the outer solar system: 2008 YB and 2005 VD.
3
N.Pinilla-Alonso1,2 (cid:63)(cid:63),(cid:63)(cid:63)(cid:63),A.Alvarez-Candal1,3,M.D.Melita4,V.Lorenzi5,J.Licandro2,6,J.Carvano7,D.
Lazzaro7,G.Carraro3,V.Al´ı-Lagoa2,6,E.Costa8,andP.H.Hasselmann7
1 InstitutodeAstrof´ısicadeAndaluc´ıa-CSIC,Apt3004,18080Granada,Spain.
e-mail:[email protected]
2 InstitutodeAstrof´ısicadeCanarias-c/v´ıaLa´cteas/n,38200LaLaguna,Tenerife,Spain
3 EuropeanSouthernObservatory,AlonsodeCo´rdova3107,Vitacura,Casilla19001,Santiago19,Chile
3 4 IAFE(CONICET-UBA),CiudadUniversitaria,IntendenteGuiraldesS/N.BuenosAires.Argentina
1
0 5 Fundacio´nGalileoGalilei-INAF,RamblaJose´AnaFerna´ndezPe´rez,7,38712Bren˜aBaja,TF-Spain
2
n 6 DepartamentodeAstrof´ısica,UniversidaddeLaLaguna,38205LaLaguna,Tenerife,Spain
a
J 7 Observato´rioNacional,COAA,RuaGal.Jose´Cristino77,20921-400RiodeJaneiro,Brazil

0
8 DepartamentodeAstronom´ıa,UniversidaddeChile,Casilla36-D,SantiagodeChile,Chile
1

]
P 16July2012;28November2012
E
ABSTRACT
.
h
p Most of the objects in the trans-Neptunian belt (TNb) and related populations move in prograde orbits with low eccentricity and
- inclination. However, the list of icy minor bodies moving in orbits with an inclination above 40 ◦ has increased in recent years.
o Theoriginofthesebodies,andinparticularofthoseobjectsinretrogradeorbits,isnotwelldetermined,anddifferentscenariosare
r considered, depending on their inclination and perihelion. In this paper, we present new observational and dynamical data of two
t
s objectsinretrogradeorbits,2008YB and2005VD.Wefindthatthesurfaceoftheseextremeobjectsisdepletedoficesanddoes
3
a notcontainthe‘ultra-red’mattertypicalofsomeCentaurs.Despitesmalldifferences,theseobjectssharecommoncolorsandspectral
[ characteristicswiththeTrojans,cometnuclei,andthegroupofgreyCentaurs.Allofthesepopulationsaresupposedtobecovered

byamantleofdustresponsiblefortheirreddish-toneutral-color.Toinvestigateifthesurfacepropertiesanddynamicalevolution
1
of these bodies are related, we integrate their orbits for 108 years to the past. We find a remarkable difference in their dynamical
v
evolutions:2005VD’sevolutionisdominatedbyaKozairesonancewithplanetJupiterwhilethatof2008YB isdominatedbyclose
1 3
encounterswithplanetsJupiterandSaturn.Ourmodelssuggestthattheimmediatesiteofprovenanceof2005VDistheintheOort
9
cloud,whereasfor2008YB itisinthetrans-Neptunianregion.Additionally,thestudyoftheirresidencetimeshowsthat2005VD
1 3
hasspentalargerlapseoftimemovinginorbitsintheregionofthegiantplanetsthan2008YB .Togetherwiththesmalldifferences
2 3
incolorbetweenthesetwoobjects,with2005VDbeingmoreneutralthan2008YB ,thisfactsuggeststhatthesurfaceof2005VD
. 3
1 hassufferedahigherdegreeofprocessing,probablyrelatedtocometaryactivityepisodes.
0
3 Key words. Kuiper belt:individual: 2008 YB3, 2005 VD - Oort Cloud - Techniques: spectroscopic - Techniques: photometric -
Methods:numerical
1
:
v
i
X 1. Introduction
r
a
Theoriginoficyminorbodiesinretrogradeorhigh-inclination
orbits is puzzling. Only a few objects in the TNb region fol-
(cid:63) PartiallybasedonobservationsmadewithESOtelescopesattheLa low orbits with an inclination larger than 40◦. Among them,
SillaParanalObservatoryunderprogramID286.C-5019(A)andwith 2008 KV , (a/q/i = 42.3AU/20.33AU/103.4◦),the first one
SOAR telescope. The SOAR telescope is a joint project of Conselho 42
discovered, moves in a retrograde orbit (Gladman et al. 2009).
Nacional de Pesquisas Cientificas e Tecnologicas CNPq-Brazil, The
Duncan & Levison (1997) explain that large semi-major axis
UniversityofNorthCarolineatChapelHillMichiganStateUniversity,
trans-Neptunianobjects (TNOs)with i up to40◦ mightbe pro-
andtheNationalOpticalAstronomyObservatory.
(cid:63)(cid:63) TheauthorthankstheInstitutoNacionaldeCieˆnciaeTecnologia ducedbyscatteringaftergravitationalencounterswithNeptune
de Astrof´ısica (INCT-A) for support through a Bolsa a Especialista during the early evolution of the solar system. But for objects
Visitante(BEV) with i > 40◦, the origin could be different. Gladman et al.
(cid:63)(cid:63)(cid:63) Presentaddress:DepartmentofEarthandPlanetarySciences,1412 (2009) discuss different scenarios that could result in the cur-
CircleDr,KnoxvilleTN37996,USA. rent orbits of these objects. Scattering from the TNb or the
1
N.Pinilla-Alonsoetal.:SurfaceCompositionandDynamicalEvolutionoftworetrogradeobjects.
Oort cloud after encounters with the giant planets is ruled out 1”. At the time of the observation, 2005 VD was at heliocen-
based on the low probability of these encounters. Instead, they tricandgeocentricdistancesof,respectively,7.44and6.60AU,
favor other scenario with an origin in a unobserved reservoir andataphaseangleof4.5o.Theobservationsweremadeusing
of large-inclination objects beyond Neptune. Recently, Brasser 2×2binning,non-sideralguiding,andexposurestimesof940,
et al. (2012), studied the origin of the orbits of Centaurs with 1200,and1480secondsfortheB,V,andRfilters,respectively.
q > 15 AU and inclinations above 70◦ based on numerical cal- Theobjectwasobservedattwoinstantsduringthenight,atair-
culations.Theyshowthat(1)Centaurswithq<15AUcanhave mass around 1.2 and 1.08. To correct for extinction, we used
anorigineitherintheOortcloudorinanunobservedreservoir severalstarsinthefieldofthephotometricLandoltstandardstar
(Gladmanetal.2009);(2)Centaurswithq >15AUmostlikely PG1323, which were observed at three different airmasses dur-
originatedintheOortcloudandwerepulleddowntotheirorig- ingthatnightwiththesameinstrumentalsetupastheasteroid.
inalorbitafterencounterswithNeptuneandUranus. Thedatareductionconsistedofoverscan,bias,andflatfield
On the other hand, there is a population of minor bodies in correctionsforallfilters.Theinstrumentalmagnitudesoftheas-
the outer solar system among which high-inclination orbits are teroidandofthestarwereobtainedthroughaperturephotometry,
dominant,theDamocloids.Jewitt(2005)introducedthistermto andtheobservationsofthestarwerethenusedtoderivetheex-
refertoobjectswithorbitslikethoseoftheHalley-typecomets tinction coefficients and zero points of the night for each filter.
(HTC),butwithoutsignsofoutgassing.Theywouldbethedor- Theresultingcalibratedmagnitudesandaveragedcolorindexes
mantnucleioftheHTC.Theirdistributionofinclinationsisvery areshowninTable1.
similartothatofthelong-periodcomets,withmostofthemhav-
ing large inclinations and several moving in retrograde orbits.
2.2. 2008YB
For simplicity, he defined this group of objects based on their 3
TisserandparameterrespectivetoJupiter1.Asithappensforthe 2.2.1. Photometry
HTC, Damocloids will have T <2. As Jewitt (2005) noted, in
J
Photometryinthevisibledomain
spite of their wide range of orbital eccentricities and inclina-
Weobserved2008YB withtheY4KCAMcameraattached
tions, Damocloids share a common characteristic, namely the 3
to the Cerro Tololo Inter-American Observatory (CTIO) 1-m
highvalueofthesetwoorbitalparameters.
telescope,operatedbytheSmall&ModerateApertureResearch
In this work, we study two retrograde ob-
TelescopeSystemconsortium2(SMARTS)duringthreeconsec-
jects with perihelion smaller than 7 AU, 2008 YB
(a/q/i = 11.65AU/5.01AU/172.901◦) and 2005 VD3 utive nights in December 28-30, 2010. All observations were
(a/q/i = 6.68AU/6.49AU/105.035◦). According to the carriedoutinphotometric,good-seeing(alwayslessthan1.2”),
conditions.About40exposurespernightweretakeninV,R,and
Minor Planet Center (MPC) definition of Centaur, these ob-
Ifilters,withtypicalexposuresof200s.
jects are Centaurs in retrograde orbits; however, according to
Basic calibration of the CCD frames was done using the
the value of their T (-1.23 and 0.14, respectively), they are
J IRAF3 package CCDRED. For this purpose, zero exposure
Damocloids. In Section 2, we present new visible colors of
framesandtwilightskyflatsweretakeneverynight.
2008 YB and the first colors of 2005 VD. We also show the
3 Because the object was transiting a crowded region of the
first spectra of 2008 YB , from 0.35 to 2.3 µm. In Section 3,
3 sky,about20o awayfromtheplaneofthegalaxy,weusedaper-
we compare these to the colors and spectra of other known
ture correction techniques to extract the magnitudes. We chose
populations of small bodies, and in Section 4, we discuss how
afewisolatedstars,typically20,tocomputethecorrectionus-
the dynamical history of these objects can be related to their
ingthemethodofcurveofgrowth(Stetson1990).Todetermine
present surface characteristics. We discuss and summarize the
thetransformationfromtheinstrumentalsystemtothestandard
resultsinSection 5.
Johnson-Kron-Cousinssystemandtocorrectforextinction,we
observedstarsinLandolt’sareasSA98(Landolt1992)multiple
timesandwithdifferentairmassesrangingfrom∼1.03to∼2.0
2. Observationsanddatareduction
andcoveringquitealargecolorrange-0.3≤(B−V)≤1.7mag.
The calibrated colors are shown in Table 1. We note that these
We present data of the two retrograde Centaurs 2005 VD and
colorswereobtainedastheaverageoverseveralmeasurements
2008 YB . We obtained photometric measurements of both, at
3
different telescopes, while spectroscopy was only obtained for takenduringtherun.
thelatterattwodifferenttelescopesandepochs:beforetheper- Since we wanted to obtain the photometric lightcurve, we
used as base the R filter observations, with alternating obser-
ihelionpassageattheTelescopioNazionaleGalileo(TNG),and
vations in the V and I filters. The lightcurves were obtained
after the passage at the ESO-Very Large Telescope. Below we
usingdifferentialmagnitudes:∆m=m −m .Outofthethree
describeindetailtheobservations. ast sta
nights, the first one could not be used for lightcurve analysis
due to the scatter in the relative magnitude product of the
2.1. 2005VD object passing close to stars in the background. The individ-
uallightcurvesobtainedfromtheusefuldataareshowninFig.1.
Photometric observations of 2005 VD using the Bessel-BVR
filters were acquired with the SOAR Optical Imager (SOI),
Photometryinthemid-infrareddomain
mounted on the 4-m SOAR telescope on Cerro Pachon, Chile.
The Wide-field Infrared Survey Explorer (WISE) surveyed
TheobservationsweremadeduringaservicemoderunonApril
the entire sky in four infrared wavelengths (3.4, 4.6, 12.0, and
3,2011.Thenightwasphotometric,withamedianseeingbelow
2 http://www.astro.yale.edu/smarts
1 TheTisserandparameterisaconstantofthemotionintherestricted 3 IRAF is distributed by the National Optical Astronomy
circularthree-bodyproblem.InparticularT providesasimpleestima- Observatory, which is operated by the Association of Universities
J
tionofthegravitationalinfluenceofJupiterintheorbitofotherobjects forResearchinAstronomy,Inc.,undercooperativeagreementwiththe
inthesolarsystem NationalScienceFoundation.
2
N.Pinilla-Alonsoetal.:SurfaceCompositionandDynamicalEvolutionoftworetrogradeobjects.
Table1.Colorindexesoftheretrogradeobjectsobserved.
Object H V (B-V) (V-R) (V-I) Observatory Reference
2005VD 14.2 23.45±0.09 0.60±0.17 0.45±0.11 SOAR thiswork
2008YB 9.5 18.231±0.001 0.82±0.01a 0.50±0.06 1.00±0.08 CTIO thiswork
3
2008YB 0.82±0.01 0.46±0.01 1.29±0.01 Sheppard(2010)
3
Sun 0.64 0.36 0.69 Hainut&Delsanti(2002)
(a)fromSheppard(2010)
22.0 µm, denoted W1, W2, W3, and W4, respectively) dur-
ing its 2010 mission (Wright et al. 2010). Following the pro-
ceduredescribedinMainzeretal.(2011),wequeriedtheWISE
Preliminary Release Single Exposure (L1b) Working Database
of the Infrared Science Archive4. This search returned WISE
data of 2008 YB , obtained on April 10-11, 2010. As recom-
3
mendedbyMainzeretal.(2011),wediscardeddatathatdidnot
meetanumberofrequirementsaccordingtotheirreportedmag-
nitudeuncertaintiesandqualityandartifactflags.Sincemostof
the data in bands W1 and W2 were rejected, only W3 and W4
magnitudeshavebeenused.Giventhatweareinterestedinrel-
ative differences within each one of the lightcurves, which are
constructedseparatelyforeachband,noneofthefurthercorrec-
tionssuggestedinWrightetal.(2010)andMainzeretal.(2011)
wereapplied.
2.2.2. Spectroscopy
TNG:Spectraobtainedbeforethepassbytheperihelion.
Visible spectra of 2008 YB were done with the 3.58m
3
TNG(ElRoquedelosMuchachosObservatory,CanaryIslands,
Spain) on January 8, 2011. The spectrograph DOLORES with
theLR-RandLR-Bgrismandthe2.0”slitwidthwasused.With
thegrismLR-Bweobtainedtwospectrawithexposuretimesof
900sec,coveringthe0.35to0.7µmspectralrange(seeTable2).
InLR-Rweobtainedthreespectraof600sec,coveringthe0.45
to 1.0 µm range. The object was shifted in the slit by 10” be-
tweenconsecutivespectratobettercorrectthefringing.Wedid
not dither in the slit when using LR-B because fringing does
notaffectthebluegrism.Thespectraobtainedwitheachgrism
wereaveragedandthenjoinedusingtheoverlappingwavelength
range.
To correct for telluric absorption and to obtain the relative
Fig.1. Lightcurves of 2008 YB . Above lightcurve obtained
reflectance,theGstarsLandolt(SA)98-978,Landolt(SA)102- 3
withtheCTIO1-mtelescopeoperatedbytheSMARTSconsor-
1081,andLandolt(SA)93-101(Landolt1992)wereobservedat
differentairmass(similartothoseoftheCentaur),beforeandaf- tium. Below, lightcurve obtained with the WISE is shown. We
estimate a rotational period based on this lightcurve of 16.388
tertheCentaurobservations,andusedassolaranalogstars.The
±0.002hr.
spectrumofeachobjectwasdividedbythoseofthesolaranalog
starsobservedthesamenightandatsimilarairmassesandthen
normalizedtounityaround0.55µm,thusobtainingthenormal-
scopewasoffsetby10(cid:48)(cid:48) inthedirectionoftheslit(positionB).
izedreflectance.Theobtainedspectrum,aftermergingwiththe
This process was repeated, obtaining four ABBA cycles up to a
near-infrared(NIR)obtainedonthesamenightasexplainedbe-
totalexposuretimeof1440seconds.Weusedtheobservingand
low,isshowninFig.2.
reductionproceduredescribedbyLicandroetal.(2001).
TheNIRspectrumwasobtainedusingthehighthroughput,
low-resolutionspectroscopicmodeoftheNear-InfraredCamera To correct for telluric absorption and to obtain the relative
andSpectrometer(NICS)attheTNG,withanamiciprismdis- reflectance, the G2 stars Landolt (SA) 102-1081, Landolt (SA)
perser. This mode yields a complete 0.8-2.5 µm spectrum. We 107-684, and Landolt (SA) 107-998 (Landolt 1992) were ob-
useda1”wideslitcorrespondingtoaspectralresolvingpower served at different airmasses during the night, before and after
R∼50,quasiconstantalongthespectrum.Theslitwasoriented the Centaur observations, and used as solar analog stars. The
at the parallactic angle, and the tracking was at the Centaur spectrum of 2008 YB3, obtained by averaging all the individ-
propermotion.Theacquisitionconsistedofaseriesof90-second ual AB pairs, was divided by the spectrum of each of the solar
exposuretimesinoneslitposition(positionA),andthenthetele- analogstars.Finally,allthedifferentspectracalibratedinwave-
lengthwereaveragedandtheresultingspectrumwasnormalized
4 http://irsa.ipac.caltech.edu/Missions/wise.html tounityaround1.0µm,thusobtainingthescaledreflectance.In
3
N.Pinilla-Alonsoetal.:SurfaceCompositionandDynamicalEvolutionoftworetrogradeobjects.
Table2.Observationalparametersofthespectroscopicobservationsof2008YB .Seetextfordetails.
3
Instrument Date UT Airmass r δ α m n×T Solar
V exp
start (AU) (AU) (◦) (secs) Analog
LRS-LRB 2011.01.08 2:25 1.44 6.49 5.71 5.7 17.8 3×600 L98-978,L102-1081,L93-101
LRS-LRR id. 2:57 1.48 id. id. id. id. 2×900 id.
NICSAmici id. 5:13 2.51-2.98 id. id. id. id. 16×90 L102-1081,L107-684,L107-998
X-shooter(UVB) 2011.01.28 2:52 1.02-1.05 6.49 5.67 5.2 17.8 4×1100 L98-978
X-shooter(VIS) id. id. id. id. id. id. id. 4×1140 id.
X-shooter(NIR) id. id. id. id. id. id. id. 4×1200 id.
Notes.FortheobservationwithX-shooterthethreedifferentexposuretimescorrespondtothesameobservation,soalltheotherparametersare
identical.FortheTNG,observationsinLRB,LRRandAmicihavebeendoneonthesamedatebutondifferentexpositions.
effects of differential refraction. Details of the observations are
presentedinTable2.Wenotethattheexposuretimesarediffer-
entforthethreearms.Theywereselectedtominimizedeadtime
duetoreadout,especiallyintheUVBandVISarms.
ThedatawerereducedandtreatedasinAlvarez-Candaletal.
(2011), but using the version 1.2.2 of the X-shooter pipeline.
Implementingthistool,thedatawerebias-anddark-subtracted,
flat-fielded,wavelength-calibrated,correctedbyatmosphericex-
tinction, and sky-subtracted. The extraction of the spectra was
made from a two-dimensional merged file, generated by the
pipeline, using IRAF. Once all spectra were extracted, we di-
videdthoseof2008YB bythestar,whichwasusedasatelluric
3
andsolaranalogstar,andcleanedthespectraofremainingbad
pixels. Since the resulting spectrum (Fig. 2) is featureless, we
appliedare-binningof5pixels,whichresultedinaspectralres-
olutionof1000.Someartifactsintroducedbythestarandpoor
telluriccorrectioncouldbeseenaround0.95and1.15µm.
Fig.2. Spectrum of 2008 YB from X-shooter and TNG. Both
3
arenormalizedtounityat0.6µm.ThespectrumfromX-shooter
3. Analysisofthedata
has been shifted 0.3 for clarity. The flat spectrum shown at the
bottomistheratiobetweenthetwospectrashowninthefigure. Oneofthegoalsofthevisiblephotometricobservationswasto
study2008YB rotationalproperties.Unfortunately,theobject
3
was crossing a crowed region of the sky and many datapoints
Fig. 2, the resulting spectra obtained by combining all the ex- hadtobediscardedduetocontaminationfromnearbystars.
tractedABofeachnightandnormalizedtojointhevisiblespec- ThetoppanelofFig.1showsthetwoindividuallightcurves
trumaround0.9µmarepresented. inthevisible,whichshowaslowincreaseinmagnitudeofabout
Around the two large telluric water band absorptions, the 0.02.Thetotaltimespanineachobservationwasabout4h.We
signal-to-noise ratio (SNR) of the spectrum is very low. Even will not give any constraint on the rotational period based on
in a rather stable atmosphere, the telluric absorption can vary these data, given that the apparent variation in brightness is of
between the object and solar analog observations, introducing theorderoftheerrorbars.
falsefeatures.Therefore,anyspectralstructureinthe1.35-1.46 TheWISEdatahaveamuchlargertimecoverage,about16h
and1.82-1.96µmregionscanproducefalsefeatures.Thereare intwofilters,andtheyarebettersuitedtosearchforarotational
also a few telluric absorption regions that are not as deep, for period. We used the method described in Harris & Lupishko
example, 0.93-0.97, 1.10-1.16, 1.99-2.02, and 2.05-2.07 µm. (1989),fittingaFourierpolynomialof3rdordertotheW3filter
Featuresintheseregionsmustbecarefullychecked. data.ThefitalsoworksfortheW4dataifweignoreonepointat
about14hrthatisnotveryreliable.Thefitteddataphasedtothis
VeryLargeTelescope(VLT):spectraobtainedafterthepass periodareshowninFig.1.Therotationalperiodweobtainedis
bytheperihelion (16.388±0.002)h.Wenotethatthisisaroughestimationofthe
Spectroscopy from the visible up to the NIR was obtained periodbasedonasparsecoverageofthelightcurve,sotheerror
withX-shooterattheESO-VLT,unit2Kueyen.X-shooterisan isprobablyunderestimated.
Echelle spectrograph able to obtain the spectrum between 0.3 The visible and infrared lightcurves cannot be phased to-
and 2.48 µm in one shot (D’Odorico et al. 2006). We observed gether due to the time span between them (almost 3 months)
2008 YB on the night of January 28, 2011 under good condi- and the different wavelength. One possible explanation for the
3
tions,usingtheslitmodeofX-shooter.Weusedthe2×1binning hugedifferencebetweenthetwoisachangeintheaspectangle
for the UVB and VIS detectors with slit widths of 1.0(cid:48)(cid:48), 0.9(cid:48)(cid:48), leadingtoanalmostpolarviewduringthevisibleobservations.
and0.9(cid:48)(cid:48),fortheUVB,VIS,andNIRarms,respectively,giving As mentioned in Section 2, the photometric data, for 2005
aresolvingpowerofabout5,000perarm. VD and 2008 YB , were averaged to extract color indexes and
3
Toremovethesolarandtelluricsignalsfromthespectra,we tobecomparedwiththedataofotherpopulationsofminorbod-
observed the star SA98-978 (Landolt 1992) with the same ob- ies.TheaveragedvisiblecolorsforbothCentaursareshownin
servationalsetupas2008YB andasimilarairmasstominimize Table.1.For2008YB ,ourcolorsareveryclosetothosepub-
3 3
4
N.Pinilla-Alonsoetal.:SurfaceCompositionandDynamicalEvolutionoftworetrogradeobjects.
Fig.4. We show the spectrum of 2008 YB obtained with X-
3
shooter,andsomeotherprimitiveminorbodies,forcomparison:
a red Centaur, Pholus (Barucci et al. 2011; Perna et al. 2010;
Fig.3. (B-V) vs (V-R) color indexes of 2008 YB (black
3 Fornasier et al. 2009; Fulchignoni et al. 2008); a grey Centaur,
square) and 2005 VD (black diamond). We include colors of
Bienor(DeMeoetal.2009;Guilbertetal.2009;Alvarez-Candal
someobjectsintheprimitiveminorbodiesforcomparison.See
et al. 2008); the average spectrum of the red and grey Trojans
text for references. Background black points are TNOs from
(Emeryetal.2011);thespectrumof2004TU ,aredcometnu-
Fulchignoni et al. (2008). Empty diamond is the average color 12
clei(Campinsetal.2006).TheretrogradeCentaurisverysimilar
of Damocloids from Jewitt (2005). Empty circle is the average
butredderthanalltheotherprimitivebodies,withtheexception
color of nuclei of Jupiter Family comets from Lamy & Toth
ofPholus,whereultra-redmatterandiceshavebeendetected.
(2009).Redandbluesquaresrepresenttheredandbluegroups
identifiedbyMelita&Licandro(2012).Asterismrepresentscol-
orsofTrojansfromEmeryetal.(2011).Inthisrepresentationwe as these values show S(cid:48)=12.86 ± 0.07 (from 0.4 to 1.35 µm),
donotdistinguishbetweenthetwogroupsidentifiedbyEmery S(cid:48)=7.41 ± 0.01 (from 11.4 to 1.79 µm), and S(cid:48)= 1.32 ± 0.15
et al. (2011) because they are not distinguishable in the visi- 2 3
(from1.99to2.3µm).Thefitofthespectrumbyastraightline
blewavelength.Weinclude2008KV ,aretrogradeTNO,from
42 issensitivetothefirstandlastpointsoftheintervalconsidered
Sheppard(2010).
andisusedtoestimatetheerrorintheslope.Itisworthbearing
inmindthattheX-shooterspectrumisobtainedsimultaneously
inthreedifferentarms.Beacusethereisasmalloverlapbetween
lishedbySheppard(2010).For2005VD,weobtainneutralcol- thespectralrangescoveredbyeacharm,mountingthecomplete
ors. Due to the faintness of this object, this is the only color spectrumistrivialanddoesnotrelyonanyfurtherobservation,
determinationfor2005VDavailableintheliterature. such as photometric data. There is a significant absorption on
Theslightlyredcolorof2008YB3isconfirmedbythechar- thespectrumabove2.3µmthatwasnotconsideredforthecal-
acteristics of its spectrum as seen in Fig. 2. This image shows culationoftheslopesbecausethispartofthespectrumcouldbe
twospectratakenwithin20days.Thetwoaresimilarthroughout degradedbythesensitivitydecayofthespectrograph.
thecompletewavelengthrange,asshownbythesmallresidues The absence of clear features in the spectrum of 2008 YB
3
resultingfromthecalculatedratioofbothspectra,whicharein- and of colors of 2005 VD in the NIR prevents us from obtain-
cludedinthesamefigure.Itisinterestingtonotethatonespec- ing further knowledge of the surface composition of these ob-
trumwastakenbeforetheperihelionpassage(at∼6.5AU),while jectshoweversomecluescanbegainedbycomparingcolorsand
theotherwastakenafterit.Wedidnotfindanyreportofactiv- spectrumwithotherminorobjectsintheoutersolarsystem.We
ityforthisobjectintheliterature.Wecheckedcarefullyallthe are particularly interested in the comparison with other primi-
imagestakenduringourobservationsanddidnotfindanytraces tiveminorbodieswithdifferentdynamicalorphysicalevolution:
ofactivity.Thissuggeststhattheactualinventoryoficesonthe Centaurs, Trojans, Damocloids, and nuclei of Jupiter Family
surfaceofthisCentaur,orclosetoit,isnotenoughtotriggeran Comets. In this comparison we also consider, when data are
activeepisodeatitsdistancetotheSun. available,theotherretrogradeobjectdiscussedinGladmanetal.
We study in detail the spectrum from X-shooter because it (2009):theTNO2008KV .
42
hasahigherSNRandresolution.Wedonotreportanyabsorp- The colors of the Centaurs follow a clear bimodal distribu-
tion feature in the whole range. The main characteristic of this tionthathasbeenobjectofstudyforyears(Peixinhoetal.2003).
spectrumisareddishslopethatchangesalongthespectralrange We represent these two groups in Fig. 3 by the average of the
covered. Sheppard (2010) computed a normalized spectral gra- colorsofthegroupsstudiedinMelita&Licandro(2012).Inthis
dientfortheopticalcolorsof9.6±0.5.Wecalculatedthereflec- work,theauthorsexplainthatthisbi-modalitycouldbeassoci-
tivity gradient S(cid:48)[%/1000 Å] over three different ranges where atedtoadifferentdegreeofprocessingofthesurface.Centaurs
the spectrum can be fitted by a straight line. We found that the in the red group have surfaces with a greater amount of ices,
slope decreases from the near-ultraviolet to the NIR domains, whereasthesurfacesoftheCentaursinthegreygrouparecov-
5
N.Pinilla-Alonsoetal.:SurfaceCompositionandDynamicalEvolutionoftworetrogradeobjects.
eredbydustmantlesthathidetheices.Dynamicalstudiesshow theyhaveplausiblesite-of-originrelationshipswiththeHalley-
how all the objects in the second group spend a larger part of type comets (Gladman et al. 2009) and the consideration of an
their life in orbits that bring them closer to the Sun, which im- extrasolar formation site is beyond the scope of this investiga-
pliesahigherdegreeofprocessingofthesurface. tion.TheorbitsofCentaursareunstable,withatypicallifetime
Another interesting group to include in the comparison is of the order of 107 years. Therefore we model their motion for
the Trojan population. Recently, Emery et al. (2011) identified 108years.
two clear groups when considering the slope in the NIR. One, We have considered the objects as point-masses under the
the redder group, is similar to the traditional D-type asteroids, gravitationalfieldoftheSunandthefourmajorplanets.Weob-
whereas the less-red group is more similar to the X-type aster- tained the initial conditions, in the form of osculating orbital
oids.Theauthorssuggestthesedifferencesarebestexplainedas elements for the planets and the minor bodies from the JPL
differencesinintrinsiccomposition,relatedtoadifferentorigin, Horizons website (http://ssd.jpl.nasa.gov/?ephemerides), corre-
ratherthananeffectofsurfacemodification.Therefore,thered spondingtoJuliandate2454101.5(seeTable3).Theequations
groupwouldbecompatiblewithanorigininthedomainofthe ofmotionwereintegratedimplementingthenumericalintegrator
volatileices,whiletheless-redgroupwouldbecompatiblewith schemeusedinBrunini&Melita(2002),whichisahybridsym-
capturebyJupiterfromnearcircularorbitsat∼5-6AU. plecticsecond-ordermethodthattreatscloseencounterswiththe
The third group we consider for comparison is the nuclei sixplanetsusingaBurlishandStoerintegratorwiththestrategy
of Jupiter Family Comets. The aspect of these objects is deter- developedbyChambers(1999).Wesettheintegrationtime-step
mined by successive epochs of activity that result in surfaces ofthesymplecticmethodequalto0.01yr.Theintegrationswere
coveredbydustmantelsthatexhaustormasktheices.Thecol- conducted into the past and stopped when the semi-major axis
ors of the comet nuclei are neutral to reddish (Lamy & Toth exceeds a value of 20,000 AU or the heliocentric distance is
2009),andtheirspectraarefeatureless.Inparticular,weshowin smallerthan0.1AUorbiggerthan100,000AU.
Fig. 4 the reddest spectrum obtained from a cometary nucleus,
162P/SidingSpring(2004TU ,Campinsetal.2006).
12
Finally,weincludeDamocloidsinthecomparison.Theori- Table 3. JPL orbital elements of 2005 VD and 2008 YB3 for
gin of Damocloids is still a matter of discussion. These ob- Juliandate2454101.5.
jects have been defined in Jewitt (2005) as the nucleus of
HalleyComets,whichhaveaprobablesourceintheOortcloud
Orbitalelement(unit) 2008YB 2005VD
(Emel’Yanenko & Bailey 1998). Dynamical theories show that 3
Semi-majoraxis(AU) 11.6573444 6.6652109
the origin of these objects could be far from the TNO popula-
Eccentricity 0.4435901 0.2488274
tion. In fact, these objects were probably formed closer to the Inclination(◦) 105.05031 172.91172
Sun than they are now and then scattered by Jupiter to high- Longitudeofnode(◦) 330.51412 178.4653
inclinationorbitswithalargesemi-majoraxisintheveryearly Argumentofperihelion(◦) 112.47615 173.03421
epochofformationofthesolarsystem.Unfortunately,wecould Meananomaly(◦) 354.51704 90.76943
notfindanyspectrumofaDamocloidinthevisibleandNIR,so
weconsidercolorsforcomparisonascanbeseeninFig.3.
Fromthecomparisonofcolorsandspectra,wefindthatthe
Theorbitalelementsusedasinitialconditionsofourcalcu-
retrograde objects have neutral- to reddish- colors that indicate
lations are given in Table 3. When the initial conditions of the
absence of ultra-red material, which is present on Centaurs in
orbits given in Table 3 are integrated into the past, we noticed
theredlobe.Thecolorsoftheretrogradeobjectsaremoresimi-
thatthedynamicalevolutionofbothobjectsisqualitativelydif-
lartothoseofsurfaceswithlowcontentofices,ascometnuclei,
ferent.Thetotaltimeofintegrationof2008YB was−20.4887
Trojans, Damocloids, and the grey group of Centaurs.There is 3
some difference between the colors of 2005 VD and the colors Myrand−6.3891Myrfor2005VD.
2008 YB , the former being more neutral than the latter. This Theinclinationoforbitof2005VDexperiencesthelargest
difference3iswellwithinthe(verylarge)errorbarsandsuggests variationsbetween120◦and180◦.Inparticularanincreaseabout
that the dust mantle covering the surface of 2005 VD could be t=5×105yr corresponds to a time-interval in which the longi-
more processed as a result of a larger residence of 2005 VD at tude of perihelion librates about 0◦. At this time, there is also
shorter perihelion distance. To check if this reasoning is sup- adecreaseineccentricity.Thisisevidencethattheorbitofthis
portedbythedynamicalhistoryofbothobjectswestudiedtheir object has evolved into a Kozai resonance (Kozai 1962). The
orbits and how they have evolved from the Oort cloud to their perihelionpenetratesintotheregionoftheterrestrialplanetsand
presentposition. isbelowthesemi-majoraxisofJupiterformostofthelifetime
span. The aphelion distance remains bounded between 300AU
and10AUuptot ≈1.5Myr.Atthispoint,thelongitudeorperi-
helionstartscirculatingandtheapheliondistancepenetratesinto
4. DynamicalModels
trans-Neptuniandistances,whereitisfoundatpresent.Theper-
Inthissection,weinvestigatethedynamicalevolutionoftheor- iheliondistanceapproximatelyrangesbetween2AUand6AU,
bits of 2005 VD and 2008 YB . At the present time, the orbits until it is expelled after a very energetic close encounter with
3
of these objects have their perihelion in the region of the gi- planetJupiter.Sucheventoccurredatapproximately5Myr,ata
ant planets. Some theories (Ferna´ndez & Brunini 2000; Dones relativedistanceof46planetaryradii.Mostofthedynamicsof
et al. 2004; Duncan et al. 1987) concur in describing the for- thisobjectisdominatedbyencounterswithplanetJupiter,with
mationsiteoftheicyminorbodiesintheOortcloudandinthe theperiheliondistancecrossinginandouttheorbitoftheplanet
TNb. However, an extrasolar origin of inner cloud comets has severaltimesinitslifetime.
alsobeenputforward(Levisonetal.2010;Brasseretal.2006). On the other hand, the orbit of 2008 YB remains quasi-
3
Weassumethatthesereservoirsarethemostplausibleimmedi- polar, with inclination values ranging between 100◦ and 110◦
atesitesofprovenanceoftheretrogradeobjectstreatedhere,as for the whole dynamical lifetime span. The aphelion distance
6
N.Pinilla-Alonsoetal.:SurfaceCompositionandDynamicalEvolutionoftworetrogradeobjects.
evolvessmoothlyfrom≈ 1000AUtoitspresentvalue.Thelon- populated spot at semi-major axis between 10AU and 20AU at
gitudeofperihelionoscillateswithdecreasingfrequencyasthe verylarge,quasi-paraboliceccentricities.A“handlingdown”of
object is transported outwards. Most of the dynamics is dom- theorbittowardsarangeofeccentricitiesclosertopresentvalues
inated by close alternate encounters with planets Jupiter and isapparent.Therefore,animmediatesiteofprovenancefromthe
Saturn, as the perihelion distance is bounded by their orbits. innerOortcloudisquitestronglyindicatedintheseplots.
Repeated close encounters finally produce the expulsion of the These diagrams give evidence of the apparent differences
object. between 2005 VD and 2008 YB in terms of dynamical evo-
3
Naturally, the orbital elements are known within a specific lution.Fromthecomparisonofthetwopanelsontherightand
uncertaintyinterval.Therefore,theorbitalevolutionoftheseob- the two panels on the left, it is apparent that 2005 VD has a
jects may not be described by the single numerical integration longerresidence-timeintheplanetaryregion,aswellasinorbits
oftheirnominalorbitalelements.Toexplorethefullsetofini- with high inclinations. For 2008 YB , the inclination is always
3
tial conditions that correspond to the presently observed orbits concentrated about its present values. But in the case of 2005
ofthesebodies,weintegratedtheorbitsof100clonesforeach VD, the inclination concentrates approximately between 120◦
asteroid.Theinitialorbitalelementsaofeachclonei,a,were and 180◦. A picture of 2005 VD coming from quasi-parabolic
i
calculatedas orbits and 2008 YB coming from the trans-Neptunian region
3
emergesnoticeably.
ai =a0+σ(a)×N(0,1), Thee-folddecreaseofthenumberofparticlesremainingin
theintegrationasafunctionoftimeimpliesasimilardynamical
where a are the nominal orbital elements used in the previ-
0 lifetime for both objects of approximately 5.5Myr. But in the
ous experiments, N(0,1) is a Normal distribution with mean
case of 2008 YB , a larger residual population exists between
equal to 0 and standard deviation equal to 1, and σ(a) is the 3
7Myr and 20Myr. As a consequence, we evaluate that the time
corresponding1σdeviationofeachelement,obtainedfromthe
spentby2005VDbetweentheplanetsisshorterthaninthecase
AstDyS database (http://hamilton.dm.unipi.it/astdys) for Julian
of2008YB .
Date2454101.5.ThesevaluesaregiveninTable4. To meas3ure the efficiency of transporting the object to the
Theintegrationswerestoppedforparticlesreachingthecri-
inner Oort cloud from each of these two groups of orbits, we
teriagivenabove.Thecalculationswerefollowedforatimescale
studiedthenumberofparticlesremainingintheintegrationthat
of 108 years in thepast, when only oneclone corresponding to
reachapheliondistancesgreaterthan100AU.Wenoticedsigni-
object2008YB3remainedandnonecorrespondingto2005VD. ficativedifferencesbetween1Myrand2Myrandbetween4Myr
and 6Myr at those time-intervals when the particles associated
with2005VDbeyond100AUexceedthoseof2008YB .Again,
Table 4. 1-sigma standard deviations of the orbital elements of 3
thissuggestsasiteofprovenancefor2005VDattheOortcloud
theorbitsof2008YB and2005VD,usedtodefinetheorbital
3 and a site of provenance, for 2008 YB , at the trans-Neptunian
elementsoftheorbitsoftheclones. 3
region.
Orbitalelement(unit) 2008YB 2005VD
3 5. DiscusionandConclusions
Semimajoraxis(AU) 0.000471 0.001605
Eccentricity 0.00002077 0.0001479
We present new data of two retrograde objects, 2005 VD and
Inclination(◦) 0.0000549 0.0003509
2008 YB . We show photometry of both objects and spec-
LongitudeofNode(◦) 0.0001475 0.003448 3
troscopyof2008YB .
Argumentofperihelion(◦) 0.002243 0.03649 3
MeanAnomaly(◦) 0.0008794 0.01661 We show lightcurves obtained for 2008 YB3 from visible
photometryandWISEdatainthefar-infrared.Bothlightcurves
areconsistent,andfromWISEdatawecangiveanestimationof
therotationalperiodof16.388±0.002hr.
In Fig. 5 we plot the residence-time in 108yr for both 2008 Colorsofthetworetrogradeobjectsaresimilar,butthereare
YB and2005VDasafunctionofsemi-majoraxis,a,andincli- somedifferencesthatcouldindicateadifferentsurfacecomposi-
3
nation,i,andalsoasafunctionofsemi-major,a,axisandeccen- tion.Whencomparedwithsolarcolors,2008YB ismoderately
3
tricity,e.Thisvalueisarepresentationofhowprobableitisto red while 2005 VD colors are more similar to the colors of the
findanobjectatacertainorbit,representedbyitsorbitalparam- Sun.
eters, at a certain time during the integration. These plots were When compared with other minor bodies in the solar sys-
generated as follows: using the output of the numerical simu- tem,wefindsimilaritiesbetweenthecolorsandspectraof2008
lationsof108 yrdurationandsamplingfrequency1/100yr,we YB and those of the red Trojans (as defined in Emery et al.
3
countedthenumberoftimesthattheorbitofeachcloneiswithin 2011),thenucleiofcomets(eithernucleiofshortperiodcomets
theboundsofacellofdimensions,1000AU/1500and180o/500 or Damocloids), and the population of grey Centaurs as shown
forthea−iplotsand1000AU/1500and1/500forthea−eplots. in Melita & Licandro (2012). For 2008 YB , there is an esti-
3
For2008YB ,theresidence-timeindicatesthattheinclina- mation of the albedo in the visible of ρ ∼0.025 ± 0.004 (V.
3 V
tion remained about its present values for most of its lifetime Al´ı-Lagoa,personalcommunication).Thisestimation,basedon
(seeFig.5left-handpanel).Oneofthemostpopulatedspotsin WISEobservation,iscompatiblewithotherlowalbedosurfaces.
thea−eplotislocatedwheretheobjectsarefoundatpresent. Withthesenewobservations,weconfirmthattheultra-redmat-
Theotheroneislocatedatsemi-majoraxeswithvaluesbetween terpresentonsomeCentaursismissingfromthesurfaceof2008
20AU and 30AU, suggesting the TNb as a plausible region of YB .Allofthis,suggeststhatthesurfaceofthisobjectlacksice
3
provenance. and is probably covered by a layer of silicates (amorphous py-
Thepicturegivenbytheresidencemapsof2005VDisqual- roxenesand/orolivine).
itativelydifferent.Fromthea−iplot,itisapparentapastasa We present colors of 2005 VD in the visible. They are af-
retrogradeobjectabouttheecliptic.Thea−eplotshowsavery fected by a larger error due to the faintness of this object but
7
N.Pinilla-Alonsoetal.:SurfaceCompositionandDynamicalEvolutionoftworetrogradeobjects.
1 60000 1 14000
0.8 50000 0.8 12000
10000
40000
0.6 0.6 8000
e 30000 e
0.4 0.4 6000
20000
4000
0.2 10000 0.2 2000
0 0 0 0
1 10 100 1000 1 10 100 1000
a (AU) a (AU)
180 200000 180 45000
180000 40000
150 160000 150 35000
120 140000 120 30000
120000
g) g) 25000
i (de 90 81000000000 i (de 90 20000
60 60000 60 15000
30 2400000000 30 510000000
0 0 0
1 10 100 1000 1 10 100 1000
a (AU) a (AU)
Fig.5. Residence maps of 2008 YB ’s orbit clones (left panels) and 2005 VD’s orbit clones (right panels). The units of the col-
3
orscalesare1.25×10−3×yr−1×AU−1×deg−1×Numberofparticles−1forthea−iplotand0.075×yr−1×AU−1×Numberofparticles−1
forthea−eplots.
clearlyshowthatitisthebluestofallofthepopulationsusedin In our case, data of both, 2005 VD and 2008 YB , suggest
3
thecomparison.Neutralcolorsintheoutersolarsystemaretyp- a lack of volatiles on the surface and a composition similar to
icallyassociatedwithhigh-albedoobjectscoveredbywaterice objectsthathavesufferedfromactivity.Thesmalldifferencebe-
(Pinilla-Alonsoetal.2008).Theyarealsotypicaloflow-albedo tweenthem,with2005VDbeingmoreneutralthan2008YB ,
3
surfaces of primitive minor objects, covered with carbons and suggests the first has a surface more processed than the latter.
silicatesopticallyinactiveatthesewavelengths.Withoutamea- This is in agreement with the results of the dynamical stud-
surementofthealbedoorobservationsintheNIR,itisnotpossi- ies. The residence times of both objects indicate that 2005 VD
bletodistinguishbetweenthesetwokindsofsurfaces.However, spends a larger lapse of time in the region of the giant planets,
basedontheorbitalcharacteristicsof2005VDwefavorthesec- reaching shorter perihelion distances in the region of the near-
ond option as the most probable composition of the surface of earth asteroids, which is in agreement with a thicker and more
2005VD. neutraldustmantlelackingicecontent.
We cannot reach a unique solution about the site of
provenance of these objects. Though retrograde orbits are
The differences in color between 2005 VD and 2008 YB
3 only compatible with a common origin in the Oort cloud, the
aresmallbutconsistentwiththeresultsofthemodelingoftheir
close evolution of these two particular objects presents some
dynamical evolution. We consider particularly interesting the differences that noticeably suggest that 2005 VD is coming
comparison of the colors of both objects with the two groups
fromquasi-parabolicorbits,while2008YB iscomingfromthe
of Centaurs that arise from the study of their colors. Trying to 3
trans-Neptunian region. In the case of 2008 YB , its evolution
disentanglethebi-modalityofthecolordistributionofthispop- 3
would be dominated by successive encounters with Jupiter and
ulation,Melita&Licandro(2012)studylinksbetweentheirdy-
Saturn,resultingintheinsertionoftheobjectfromtheTNbora
namicalhistoryandtheirsurfacephysicalproperties.Theyfind
moredistantreservoiratQ ∼70AUintoitspresentorbit,while
that the differences in colors could be related to different ther-
thedynamicsof2005VDisdominatedbycloseencounterswith
malprocessingoftheirsurfaces.Theobjectsinthegreylobeof
planetJupiterandalargerresidencetimeintheplanetaryregion.
the Centaurs have probably spent more time at short distances
fromtheSunand,asaresult,theypassedbymoreactiveepochs
Insummary:
thantheobjectsintheredlobe.Basedonthis,theauthorspro-
posedactivityisprobablythereasonwhythecolorsofthegrey
Centaurs are more similar to those of comet nuclei and other – 2005VDand2008YB showcolorsandspectra(inthecase
3
objectscoveredbymantlesofdust. of2008YB )similartothecolorsandspectraofotherpopu-
3
8
N.Pinilla-Alonsoetal.:SurfaceCompositionandDynamicalEvolutionoftworetrogradeobjects.
lationsofminorbodieslikelycoveredbymantlesofsilicates Harris,A.W.&Lupishko,D.F.1989,inAsteroidsII,ed.R.P.Binzel,T.Gehrels,
anddepletedofices. &M.S.Matthews,39–53
– 2005VDismoreneutralandappearsontheblueendofthe Jewitt,D.2005,AJ,129,530
Kozai,Y.1962,AJ,67,591
color-colorplot,asshowninFig.3,while2008YB ,isred-
3 Lamy,P.&Toth,I.2009,Icarus,201,674
dish. This suggests that the surface of the former is more Landolt,A.U.1992,AJ,104,340
processed. Levison,H.F.,Duncan,M.J.,Brasser,R.,&Kaufmann,D.E.2010,Science,
– Thestudyofintegrationsoftheirorbitsshowsthattheseob- 329,187
Licandro,J.,Oliva,E.,&DiMartino,M.2001,A&A,373,L29
jects have been retrograde for most of their histories. From
Mainzer,A.,Grav,T.,Masiero,J.,etal.2011,ApJ,736,100
our studies the injection of their orbits from the Oort cloud
Melita,M.D.&Licandro,J.2012,A&A,539,A144
isprobablydifferent:1)For2008YB3,theTNbasaplausi- Peixinho,N.,Doressoundiram,A.,Delsanti,A.,etal.2003,A&A,410,L29
ble region of provenance is suggested; 2) For 2005 VD, an Perna,D.,Barucci,M.A.,Fornasier,S.,etal.2010,A&A,510,A53
immediate site of provenance from the inner-Oort cloud is Pinilla-Alonso,N.,Licandro,J.,&Lorenzi,V.2008,A&A,489,455
Sheppard,S.S.2010,AJ,139,1394
quitestronglyindicated.
Stetson,P.B.1990,PASP,102,932
– Fromthestudyofthesurfacecharacteristicsanddynamical Wright,E.L.,Eisenhardt,P.R.M.,Mainzer,A.K.,etal.2010,AJ,140,1868
evolutionofbothobjects,theyhaveprobablysufferedfrom
activity related to ices’ volatilization that resulted in a sur-
face covered by a mantle of silicates. The larger residence-
time of 2005 VD in the planetary region of the giant plan-
etswithpossiblesemi-majoraxisbetween3and10AUisa
plausibleexplanationfortheextremecolorofthisobjectin
thevisible.
Acknowledgements. NPAwouldliketothanktheInstitutoNacionaldeCieˆnciae
TecnologiadeAstrof´ısicadoBrasil,financedbyCNPqandFAPESP,foritssup-
portwhenvisitingasSpecializedResearcher.MDMacknowledgescontribution
byANPCyTthroughPICT218/2007andCONICETPIP11220090100461/2010.
JMC, DL, and PHH were supported by diverse grants and fellowships by
CNPq,FAPERJ,andCAPES.E.C.acknowledgessupportbytheFondoNacional
de Investigacio´n Cient´ıfica y Tecnolo´gica (proyecto No. 1110100) and the
Chilean Centro de Excelencia en Astrof´ısica y Tecnolog´ıas Afines (PFB 06).
Part of this work is based on observations made with the Italian Telescopio
NazionaleGalileo(TNG)operatedontheislandofLaPalmabytheFundacio´n
Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish
Observatorio del Roque de los Muchachos of the Instituto de Astrofisica
de Canarias. This publication makes use of data products from the Wide-
field Infrared Survey Explorer, which is a joint project of the University of
California,LosAngeles,andtheJetPropulsionLaboratory/CaliforniaInstitute
ofTechnology,fundedbytheNationalAeronauticsandSpaceAdministration.
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